This invention relates to a remote control signal repeater for transmitting a remote control signal and, more particularly, to a remote control signal repeater having a bandbass filter and a level slicer capable of preventing an erroneous operation of a remote controlled device due to noise mixed in a remote control signal.
A remote control signal repeater is a device for transmitting a remote control signal such as an infrared signal to a separate location such as a separate room. A general structure of a repeater is shown in FIG. 2. In FIG. 2 is shown an example in which an infrared remote control signal is transmitted from a room 12 to a room 14. A repeater 10 includes, a light receiving element 16 such as a photodiode, an amplifier 17, a bandpass filter 18, a level slicer 19 and a light emitting element 20 such as an infrared light emitting diode. The light receiving element 16 is provided in the room 12 and receives infrared remote control signals 24, 25 and 26 outputted in the form of intermittent pulses from different types of infrared remote controllers 21, 22 and 23. The received signals are amplified by the amplifier 17 and a necessary band of the received signals is passed by the bandpass filter 18. The bandpass filter 18 is provided for eliminating noise occurring in frequencies on either side of the necessary band for transmitting the infrared remote control signals. Noise mixed in a signal of the band passed by the passed bandpass filter 18 is cut off by the level slicer 19. The light emitting element 20 is provided in the room 14 and driven by an output signal of the level slicer 19 to output infrared pulse remote control signals 24', 25' and 26' which coincide with the infrared remote control signals 24, 25 and 26. These infrared remote control signals 24'-26' are received by light receiving sections of main devices 28, 29 and 30 provided in the room 14 to perform operations as instructed by the infrared remote controllers 21-23. In this manner, the devices 28-30 can be controlled from the separated room 12 by using the infrared remote controllers 21-23.
In the room 12, there exist various types of noise due to light (noise light) occurring from various sources such as light generated by a fluorescent lamp. Also another type of noise is generated by the light receiving element 16 and the amplifier 17. The light receiving element 16 provided for receiving the infrared remote control signal is made of a photodiode and picks up the noise light because the photodiode 16 has an ability to convert not only infrared but also visible light. The bandpass filter 18 eliminates noises occurring in the frequencies on either side of the necessary band.
A bandpass filter circuit is generally constructed of an LC resonant circuit as shown in FIG. 3. Since a bandpass filter of a remote control signal receiving circuit provided in each device to be controlled by a remote controller has only to be operated by a remote control signal of a single carrier frequency, a signal of a frequency other than the carrier frequency can be accurately eliminated and an accurate operation of each device can thereby be ensured by setting Q in the frequency characteristic of the LC resonant circuit at a high value as shown in FIG. 4.
In the case of a repeater, however, various remote control output signals (having generally carrier frequencies ranging from 30 kHz to 40 kHz) must be transmitted and, therefore, the bandpass filter 18 must have a sufficient band width to cope with such various remote control signals for this purpose, the bandpass filter 18 of a conventional repeater is made of a combination of plural CR filters and thereby provides a characteristic as shown in FIG. 5 which is obtained by superposing 6 dB/oct curves to obtain the necessary band width of 30 kHz to 40 kHz.
The bandpass filter 18 having the characteristic of FIG. 5, however, has the disadvantage that, since the attenuating characteristic of the filter in frequencies other than the necessary band is rather gradual, response in the unnecessary frequency region increases and the bandpass filter 18 therefore tends to pick up noises resulting in deterioration of the signal-to-noise ratio and increased possibility of an erroneous operation of the devices receiving remote control signals.
It is, therefore, an object of the invention to provide a remote control signal repeater having an improved bandpass filter circuit which can secure a necessary band width and yet realize a sharp attenuating characteristic in frequencies other than the necessary band and thereby prevent an erroneous operation of devices due to noises.
Reverting to FIG. 2, high frequency components of noise light and noises generated by the light receiving element 16 and the amplifier 17 exist not only in the outside of the carrier frequency of the remote control signals but also in the vicinity of the carrier frequencies of the remote control signals and the bandpass filter 18 cannot eliminate these noises because they exist inside of the necessary band of the bandpass filter 18, Since, as described above, the band of the bandpass filter 18 of the repeater is set at a relatively large width for the requirement of transmitting various remote control signals having generally carrier frequencies ranging from 30 kHz to 40 kHz, a significant amount of noise is outputted by the repeater 10 without being eliminated by the conventional bandpass filter 18.
For eliminating noises which have thus passed the bandpass filter 18, the level slicer 19 is provided. The level slicer 19 inhibits outputting of a signal below a predetermined level, regarding it as a noise. For enabling the level slicer 19 to operate accurately, a noise of a large amplitude outside of the necessary band must be eliminated by the bandpass filter 18.
A conventional level slicer utilizes a rising characteristic of a transistor or a diode. An example of such conventional level slicer is shown in FIG. 14. A received signal which has been attenuated in frequencies outside of the necessary band by the bandpass filter 18 (FIG. 2) and amplified to some extent is applied to an input terminal 132. This received signal is attenuated in its direct current component by a capacitor C110 and supplied to a transistor Q110 without bias and led to an infrared LED (light emitting diode) 20. The operation of the level slicer 19 of FIG. 14 is shown in FIG. 16. The transistor Q110 is turned on and drives the infrared LED 20 when an input signal is above a junction voltage (about 0.6 V) thereof. By cutting off a signal below the junction voltage in this manner, a noise in the band is eliminated.
In the conventional level slicer 19 shown in FIG. 4, it would be ideal if the voltage-current characteristic of the junction voltage of the transistor Q110 was one shown by the dotted line in FIG. 15. In actuality, however, the voltage-current characteristic is one shown by the solid line in FIG. 15 and this is disadvantageous in that a remote control signal cannot be separated clearly from a noise.
Moreover, since the slicing level is a fixed value in the conventional level slicer, a sufficient effect of the slicer cannot be obtained when the level of a noise is excessively large. If the slicing level is set at a large level to cope with such noise of a large level, an infrared remote control signal coming from a distant remote controller will not be able to exceed the slicing level, so that an allowable distance of a remote controller will have to be shortened.
It is, therefore, another object of the invention to provide a-remote control signal repeater having an improved level slicing circuit capable eliminating a noise within the necessary band effectively and thereby preventing an erroneous operation of devices due to noises.